With the increasing processing speed of large-scale integrated circuits (LSIs), decoupling capacitors have been increasingly used in recent years. To improve high-frequency tracking performance of a decoupling capacitor, it is necessary to reduce inductance between the decoupling capacitor and an LSI. For this purpose, the decoupling capacitor is directly disposed in the LSI, which is connected via bumps to the decoupling capacitor.
Examples of thin-film capacitors used for decoupling include a thin-film capacitor disclosed in Patent Document 1. The disclosed thin-film capacitor will be described with reference to FIG. 10.
A thin-film capacitor 100 includes a lower electrode 102, a dielectric thin film 103, and an upper electrode 104 that are disposed on a substrate 101 in this order. Contact pads 107a and 107b are connected to the lower electrode 102 and the upper electrode 104, respectively. Bumps 108a and 108b for making an electrical connection to an LSI, to a mounting board, and the like are disposed on the contact pads 107a and 107b, respectively. Additionally, a protective insulating layer 106 of resin material, such as polyimide, is provided for absorbing mechanical stress from the bumps 108a and 108b. A barrier layer 105 of nonconductive inorganic material is disposed between a capacitor unit (including the lower electrode 102, dielectric thin film 103, and upper electrode 104) and the protective insulating layer 106. The barrier layer 105 protects the dielectric thin film 103 from being adversely affected by hydrogen ions dissociated from H2O produced by a dehydration condensation reaction that occurs when polyimide is cured.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-214589
In the invention described in Patent Document 1, the protective insulating layer 106 is provided to absorb mechanical stress from the bumps 108a and 108b. The protective insulating layer 106 serves as a shock absorber which is effective, to some extent, against stress in the horizontal direction (which is parallel to the primary surface of the substrate 101 and is the lateral direction in the drawing) on the bumps 108a and 108b. However, the shock-absorbing effect of the protective insulating layer 106 against stress having a component in the vertical direction (which is perpendicular to the primary surface of the substrate 101 and is the longitudinal direction in the drawing) is not necessarily sufficient.
Here, there will be described a mechanism in which stress having a component in the vertical direction acts on the bumps 108a and 108b. A Si substrate typically used to form a thin-film capacitor has a coefficient of linear expansion of 2 to 3 ppm/° C. On the other hand, a resin multilayer substrate has a coefficient of linear expansion of several tens of ppm/° C., which is much greater than that of the Si substrate. Hence, when the thin-film capacitor is mounted on the resin multilayer substrate and a temperature change occurs, either of the Si substrate and the resin multilayer substrate warps due to the difference in coefficient of linear expansion between these substrates.
Whether the Si substrate or the resin substrate warps depends on the thicknesses and Young's moduli of these substrates. For example, in the case where the Si substrate is more easily deformed than the resin substrate, when the thin-film capacitor is mounted with solder bumps on the resin substrate and cooled, the resin substrate contracts to a greater degree than the Si substrate. Therefore, the thin-film capacitor is deformed such that its surface on which no bumps are formed is raised. This deformation causes large tensile stress to be generated in bumps located in the center of the thin-film capacitor. At the same time, this deformation causes shearing stress, which is stress in the direction along the surface of the Si substrate, to be generated in bumps located near the periphery of the thin-film capacitor. On the other hand, in the case where the resin substrate is more easily deformed than the Si substrate, the resin substrate is deformed, when cooled, such that its surface on which the thin-film capacitor is not mounted is depressed. This deformation causes large tensile stress to be generated in outer bumps.
Referring to FIG. 10, when tensile stress in the upward direction in the drawing is generated in the bump 108b, since the bonding strength of the interface between the bump 108b and the contact pad 107b and the bonding strength of the interface between the contact pad 107b and the upper electrode 104 are relatively strong, the upper electrode 104 is pulled upward. Similarly, when shearing stress is generated in the bump 108b, the upper electrode 104 is pulled in the lateral direction. At the same time, the bonding strength of the interface between the upper electrode 104 and the dielectric thin film 103 is relatively weak, because they are made of different materials (that is, the upper electrode 104 is made of metal, while the dielectric thin film 103 is an oxide). This causes separation in the interface between the upper electrode 104 and the dielectric thin film 103, leads to rupture of the upper electrode 104, and may significantly damage the functions of the capacitor. Even if the separation does not occur in the interface, large residual tensile stress in the interface adversely affects the reliability of the capacitor.
Substrate warpage, which causes such tensile stress, is particularly significant when lead-free solder having a high reflow temperature is used as a material of bumps. Since use of lead-free solder has increased in recent years because of environmental concerns, the above-described problems need to be addressed urgently.
Although, for illustrative purposes, there has been described the case where the thin-film capacitor is mounted on the resin substrate, similar problems arise when the thin-film capacitor is mounted on a ceramic substrate. A ceramic substrate has a coefficient of linear expansion smaller than that of a resin substrate, but has a Young's modulus higher than that of the resin substrate. Therefore, it is still true that large tensile stress occurs in bumps.
Additionally, when a sapphire substrate (having a coefficient of linear expansion of about 8 ppm/° C.) or a quartz substrate (having a coefficient of linear expansion of about 0.5 ppm/° C.) is used as a substrate of the thin-film capacitor, the above-described problems arise due to the difference in coefficient of linear expansion between the substrate used and a mounting board.
When the thin-film capacitor is used as a decoupling capacitor for a microprocessing unit (MPU) of a computer or the like, it may be necessary to increase the equivalent series resistance (ESR) of the thin-film capacitor. Generally, a plurality of capacitors is used as decoupling capacitors for an MPU, and a capacitor having a capacitance greater than that of the thin-film capacitor is disposed at a position more distant from the MPU than the thin-film capacitor is. Such simultaneous use of capacitors having different capacitances and inductances is known to cause a phenomenon in which impedance increases at a specific frequency. It is also known that such phenomenon can be prevented by increasing the ESR of the thin-film capacitor. Since the thin-film capacitor has a small ESR because of its structure, a relatively thick resistive film is disposed between electrodes to increase the ESR. However, insertion of the thick resistive film causes the electrodes of the capacitor to be stressed. This is attributed to the fact that since the deposition stress of a nitride film typically used as a resistor is large and the Young's modulus of the resistor is high, a large stress is applied to the electrodes.